The present invention is directed to a technique for investigating characteristics of subsurface earth strata. More particularly, the present invention relates to a technique for determining the electrical conductivity of earth formations with an induction well logging tool.
Various techniques for investigating the conductivity of subsurface earth formations have been developed. One such technique, referred to as induction logging, was first introduced in 1949 by Henri G. Doll, "Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Base Mud," Journal of Petroleum Technology, Vol. 1, No. 6, June 1949, pp. 148-162. U.S. Pat. No. 2,582,314 was issued Jan. 15, 1952 and related to the basic Doll induction logging system.
Briefly, an induction logging tool employs at least one transmitter/receiver coil pair. Typically, the transmitter coil is excited by an oscillating current at a frequency on the order of 20 KHz. The resultant magnetic field generates eddy currents in the earth formation which, in turn, produce a second magnetic field. The second magnetic field generates voltage signals across the receiver coil, which are then processed to obtain values for the earth conductivity (or the inversely related resistivity). The conductivity values are recorded as a function of the tool depth.
Over the years attempts have been made to improve the accuracy of conductivity measurements obtained with an induction logging tool. For example, considerable efforts have been made in an attempt to minimize the effects on the conductivity measurement caused by environmental factors such as the shoulder effect and the skin effect. Briefly, the shoulder effect is the distortion of measured data at the measure point caused by different conductivities in formation beds remote from the measure point. The skin effect relates to distortion of the measurement due to the detection of magnetic fields resulting from eddy currents in the formation which, in turn, were produced by magnetic fields generated by other eddy currents in the formation. The design of the coil array can help reduce the impact of the environmental effects. However, such an approach to reducing undesirable effects limits flexibility in coil array design.
In order to avoid unduly constraining the design of the induction logging coil array, many attempts at reducing environmental effects on the conductivity measurement focus on the techniques employed in processing the voltage signals detected by the receiver coil. The technique described by Doll, which may be described as the geometric factor theory, does not account for the skin effect. One processing technique intended to correct for the skin effect is disclosed in U.S. Pat. No. 3,147,429 issued Sep. 1, 1964 to James H. Moran. The induction logging tool disclosed by Moran utilizes a phase sensitive detector keyed to the transmitter coil current to obtain both an in-phase receiver signal (the "V.sub.R signal") and a quadrature phase detector signal (the "V.sub.X signal"). In the disclosed embodiment, the V.sub.X signal is modified by the V.sub.X signal to obtain an indication of the formation conductivity which should have reduced skin effect. This technique, however, has various shortcomings. For example, the quadrature signal often exhibits instability in certain formations. This instability may be difficult to estimate and leads to questionable accuracy of the final conductivity measurements.
Other processing techniques have been described in the technical and patent literature. A technique often designated "phasor processing" is described in U.S. Pat. Nos. 4,467,425 and 4,471,436 to Richard T. Schaefer et al, and U.S. Pat. No. 4,513,376 to Thomas D. Barber. Additional techniques for obtaining conductivity measurements with an induction logging tool are disclosed, for example, in Suresh G. Thadani et al., "Deconvolution With Variable Frequency Induction Logging Systems," SPWLA 24th Annual Logging Symposium, Paper II, June 1983; J. H. Moran et al, "Induction Logging--Geometrical Factors With Skin Effect", The Log Analyst, Vol. 23, No. 5, Nov.-Dec. 1982, pp. 4-10; B. Anderson, "Induction Sonde Response in Stratafied Media," The Log Analyst, Vol. 24, No. 1, Jan.-Feb. 1983, pp. 25-31; O. Serra, "Diagraphies Differees--Bases de l'Interpretation. Tome 1: Acquisition des Donnees Diagraphiques," Bulletin des Centres de Recherches Exploration-Production Elf-Aquitaine, Memoire 1, 1979, p. 92; C. F. George et al, "Application of Inverse Filters to Induction Log Analysis," Geophysics, Vol. 24, No. 1, February 1964, pp. 93-104; and T. D. Barber et al, "Introduction to the Digital Dual Induction Tool," Trans. of the 58th Annual Tech. Conf. and Exhibition of the SPE of AIME, SPE Paper No. 12049, Oct. 5-8, 1983.
All of the induction logging techniques noted above have various shortcomings which reduce their effectiveness in certain conditions. Accordingly, there is a need for an induction well logging system which provides improved conductivity measurements over a large range of formation conductivities.